This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The production of hydrocarbons, such as oil and gas, has been performed for numerous years. To produce these hydrocarbons, a wellbore is typically drilled in intervals with different casing strings installed to reach a subsurface formation. The casing strings are installed in the wellbore to prevent the collapse of the wellbore walls, to prevent undesired outflow of drilling mud into the formation, and/or to prevent the inflow of formation fluid into the wellbore. Because the casing strings for lower intervals pass through already installed casing strings, the casing strings are formed in a nested configuration that continue to decrease in diameter in each of the subsequent intervals of the wellbore. That is, typically casing strings in the lower intervals have smaller diameters to fit within the previously installed casing strings. Alternatively, the expandable casing strings may be utilized within the wellbore. However, the expandable casing strings are typically more expensive and increase the cost of the well.
The process of installing casing strings involves tripping/running the casing string and cementing the casing string, which is time consuming and costly. With the nested configuration, the initial casing strings have to be sufficiently large to provide a wellbore diameter that is able to be utilized for the tools and other devices. With subsurface formations being located at greater depths, the diameter of the initial casing strings are relatively large to provide a final wellbore diameter useable for the production of hydrocarbons. Large wellbores increase the cost of the drilling operations because the increased size results in increased cuttings, increased casing string size and costs, and increased volume of cement and drilling mud utilized in the wellbore.
Accordingly, various processes are utilized to reduce the diameter of casing strings installed within the wellbore. For example, some processes describe modifying the drilling mud to install fewer different casing strings within the wellbore. A drilling mud is utilized to remove cuttings and provide hydrostatic pressure to the subsurface formation to maintain drilling operations for a well. The weight or density of the drilling mud is typically maintained between the pore pressure gradient (PPG) and the fracture pressure gradient (FG) for drilling operations. However, the PPG and FG often vary along with the true vertical depth (TVD) of the well, which present problems for maintaining the weight or density of the drilling mud. If the density of the drilling mud is below the PPG, the well may kick. A kick is an influx of formation fluid into the wellbore, which has to be controlled before drilling operations may resume. Also, if the density of the drilling mud is above the FG, the drilling mud may be leaked off into the formation. The leakage may result in lost returns or large volumes of drilling mud loss, which has to be replaced for the drilling operations to resume. Accordingly, the density of the drilling mud has to be maintained within the PPG and FG to continue drilling operations that utilize the same size casing string.
Accordingly, drilling operations may utilize variable density drilling mud to maintain the density of the drilling mud within the PPG and FG for the wellbore. See Intl. Patent Application Publication No. WO 2006/007347. To reduce the number of intermediate casing strings utilized within the well, the variable density drilling mud may include various compressible particles to provide a drilling mud that operates within the PPG and FG. Because the drilling operations may be continuous, the compressible particles may have to circulate within the wellbore one or more times. As such, there is a need for a method and apparatus for managing the compressible particles that are utilized within the variable density drilling mud.
Other related material may be found in at least U.S. Pat. Nos. 3,174,561; 3,231,030; 4,099,583; 4,192,392; 5,881,826; 5,910,467; 6,156,708; 6,415,877; 6,422,326; 6,497,289; 6,530,437; 6,588,501; 6,739,408; 6,953,097; U.S. Patent Application Publication No. 2004/0089591; U.S. Patent Application Publication No. 2005/0023038; U.S. Patent Application Publication No. 2005/0113262; U.S. Patent Application Publication No. 2005/0161262; and Intl. Patent Application Publication No. WO 2006/007347.